1. Field of the Invention
The present invention relates to a method and a device for controlling the amplitude of the wavelength spectrum of ultra-short light pulses emitted by multiple passage laser amplifiers.
2. Description of the Prior Art
This method is notably applied to laser amplifiers with multiple passages of light in an amplifier crystal, the amplified light pulses of which have a duration between a few femtoseconds and a few picoseconds.
A laser amplification chain comprises an oscillator and one or more amplifiers; in the case of short pulse laser chains, the oscillator should have a large bandwidth in order to generate a short reference pulse; the bandwidth of said reference pulse is reduced by the amplifiers as a result of an effect of gain reduction on the edges of the spectrum of said reference pulse; this effect contributes to reducing the wavelength spectrum of the pulse during the multiple passages in the amplifier chain; the output pulse of the amplifiers is therefore longer than said reference pulse.
Generally speaking, it is known that to combat this effect of gain reduction, the amplitude of the spectrum of the reference pulse must be reduced at the centre of the bandwidth; this may be achieved by means of a fixed or programmable filter placed between the oscillator and the first amplifier or in the amplifier itself when the latter is of the type with multiple passages of the light pulse in the amplifier crystal.
A reduction in the amplitude of the spectrum of the initial pulse at the centre of the bandwidth, upstream from the amplification, should be all the more significant since post-amplification is large.
It is found that such filtering of the spectrum of the initial pulse, even if it is thereby possible to combat the reduction in bandwidth and to extend the spectrum of the amplified pulse, moreover introduces undesirable phase effects due to the fact that the light propagation velocity in the amplifier crystal is slightly slower when the amplified energy is larger (effect due to the non-linear index of the amplifier crystal).
Thus, the wavelengths of the light corresponding to both ends of the spectrum, have slower propagation velocities in the amplifier crystal than those located in the central portion of said spectrum.
This results in a 3rd order phase distortion introduced by the strong amplitude modulation of the filtered spectrum.
When the filter is placed in the cavity of an amplifier with multiple passages, the light pulse passes N times through the amplifier crystal before being extracted; in this configuration, reduction in the amplitude of the spectrum of the reference pulse, at the centre of the bandwidth, may be the Nth root of the amplitude reduction corresponding to a single passage.
Indeed, if g1(λ) is the curve of the gain versus the wavelength corresponding to the first passage, g2(λ) the curve of the gain corresponding to the 2nd passage, . . . , gN(λ) the curve of the gain corresponding to the Nth passage, the curve of the total gain corresponding to N passages is:G(λ)=g1(λ)·g2(λ) . . . gN(λ),and if the N passages are gain identical, g(λ), then G(λ)=[g(λ)]N. Thus, a 10 decibel reduction at the centre of the curve of the gain may be obtained by 20 passages with 0.5 decibel in a multiple passage amplifier.
This solution however imposes that the filter introduced into the cavity of the amplifier, has an insertion loss as low as possible in order to maintain the gain of said cavity.